What’s the main reason for data loss (technical background, composition)?

A CD/DVD is made up as follows: on a substrate of polycarbonate the writing layer is put, with a metallic reflection layer added afterwards. Both these layers are sealed with a fourth layer of special clear varnish on CD-Rs or bonding glue on DVDRs (see drawing). The media is written to and read using a laser beam that shines through the plastic substrate to the recording layer and is beamed back by the reflective layer.

There are many reasons why media may become unreadable. These include, in the first instance, damage caused by the user through scratches, fingerprints or other day to day handling damages. In second place, inappropriate storage - e.g. in a car (heat) or in direct sunlight - plays an important role, causing the recording layer to decompose. Thirdly, the original burn quality is often insufficient, due to faulty burners, unspecified or too-high burn speeds, makes of media that have not been tested by the burner manufacturer, etc. Faults in the manufacture of the media can also play a role. These include inadequate sealing, allowing atmospheric oxygen to reach the recording/reflective layer, causing it to decompose/corrode (rust).